Vibratory feeders for small parts such as screws, nuts, plastic pieces and so on are generally AC powered and electromechanically tuned to either a 60 or 120 cycle per second frequency. For bowl-type feeders, a bowl for the parts which includes a spirally ascending track interiorally about its circumference is mounted on an intermediate portion which rests on a base. The intermediate portion is coupled to an AC power source through a power control and is electromagnetically tuned to vibrate the bowl. Vibration of the bowl causes the parts to move upwardly along the spiral track.
These parts then move to a machine feed track where they are temporarily stored for feeding into an assembly machine. Since the assembly machine is usually part of an assembly line, it is desirable to have the assembly machine operating at a constant rate not constrained by the rate at which parts may be fed. Thus the vibratory feeder is controlled to feed parts at a faster rate than that which the parts are accepted by the assembly machine. A sensor detecting a parts backlog shuts off the vibratory feeder when a sufficient number of parts are fed to the machine feed track.
Since the combination of the vibratory feeder, machine feed track, and parts backlog detecting sensor constitutes a simple feedback loop, it is desirable to introduce a delay so that the vibratory feeder is turned on and off at a rather slow rate rather than at a high rate which would occur with a very short delay. For this purpose the prior art systems have used a separate sensor switch, delay circuits, and power control circuits, which are connected by signal cables. The delay circuits present either an on delay time between the instant that the sensor switch indicates the absence of a backlog and the power control circuit turns on the vibratory feeder, or an off delay between the time that a parts backlog is detected and the vibratory feeder control shuts off the vibratory feeder. Alternatively, the on delay circuit and the off delay circuit may be cascaded to form a dual delay circuit in which there are both on delays and off delays.
Prior art dual delay controllers have used individual potentiometers to set the on delay and the off delay. This is true even though it is advantageous to have the on delay and the off delay time generally equal. In practice it is found that it is troublesome to have to adjust both of the potentiometers to approximately the same delay.
There is also a problem with the prior art in that the signal cables connecting the sensor switch, delay circuits, and power control circuits are suceptible to the pick-up of electromagnetic noise such as occur in an industrial environment.